Using existing topology to model details and grooves
I would do it this way: First model the general shape of the gun with no worry for the grooves. Once your good, begin to draw the grooves lines with the knife. Keep using the knife to make sure you have mainly quads and not tris or ngons, then bevel the grooves, and extrude inwards. You'll have to bring some corrections of course. You could even separate the grip in several different meshes as anyway it will be hidden by the grooves.